1. Field of the Technology
The technology relates to a developing unit used in an image forming apparatus such as a copier, printer, facsimile machine or the like using electrophotography as well as relating to an image forming apparatus including this developing unit. The technology in particular relates to a carrier collecting roller provided for a developing unit in a high-speed processing image forming apparatus.
2. Description of the Prior Art
Recent image forming apparatuses such as copiers, facsimile machines, printers, so-called multifunctional machines having these functions use a dual-component developing process using a dual-component developer consisting of a toner and a magnetic carrier. The dual-component developing process is an image forming process for performing image formation by electrostatically adhering the toner that has been tribo-electrified by the magnetic carrier to the electrostatic latent image formed on the photoreceptor.
The magnetic carrier used in the dual-component developing process has a function of forming a magnetic brush by charging the toner by electrifying itself with the opposite polarity to that on the toner so as to form a magnetic brush to thereby perform development. Usually, this carrier itself will not adhere to the photoreceptor.
However, since the carrier is charged with the opposite polarity to that on the toner, there have been cases where the carrier adheres to the background area and areas other than the background but having higher potentials than the latent image potential levels. Once the carrier adheres to the photoreceptor, the carrier lowers the transfer medium's adhesion to the photoreceptor during transfer, inducing transfer image degradation such as white voids and the like.
The leftover toner remaining on the photoreceptor surface after transfer of the toner image is usually removed by a cleaning member of a cleaning unit so as to clean the photoreceptor drum surface. However, if the carrier remains on the photoreceptor surface during the cleaning, there is a fear that the carrier damages or scratches the photoreceptor and induces degradation of the cleaning member, hence shorten the lives of these components.
In the field of image forming apparatuses, there is a trend toward high speed and high resolution. To achieve this, the carrier is demanded to be smaller in size. However, as the carrier becomes smaller in size, the carrier becomes liable to adhere to the photoreceptor during development. In addition, the enhancement of the speed of printing in the image forming apparatus means increase in the rotational speed of the photoreceptor drum, and this has caused difficulties in collecting and conveying the carrier from photoreceptor drum. To deal with this, there are many developing unit configurations that include a carrier collecting roller arranged on the downstream side of the developing roller.
As one example of a developing unit that facilitates collection of the carrier from the photoreceptor drum by using such a carrier collecting roller has been disclosed in Japanese Patent Application Laid-open 2006-215402 (patent document 1).
That is, patent document 1 discloses an image forming apparatus using a developing unit that develops the electrostatic latent image formed on the photoreceptor by forming a magnetic brush of a developer consisting of a carrier and a toner on a developing roller that includes fixed magnets therein. In this image forming apparatus, a magnetized carrier collecting roller is arranged downstream of the developing roller while a magnet is laid out at a position opposing this carrier collecting roller inside the photoreceptor so that this magnet forms a magnetic field that is parallel with the tangent direction of the outer periphery of the photoreceptor. In this image forming apparatus, penetration of the carrier into the transfer device is prevented by attracting the developer that has come along the photoreceptor from the developing area to the carrier collecting roller by the effect of the magnetic field formed by the magnets disposed inside the photoreceptor and in the carrier collecting roller, to thereby prevent occurrence of white voids in the image and damage to the photoreceptor.
In this case, the developing unit disclosed in patent document 1, the magnetic poles of the magnet of the carrier collecting roller are disposed so as to oppose the photoreceptor. That is, the magnetic field created by the magnet disposed inside the photoreceptor and the magnet in the carrier collecting roller attracts surplus developer to the carrier collecting roller, to thereby prevent occurrence of white voids in the image and damage to the photoreceptor. However, this disclosure includes no reference to the effect as to whether the carrier can be collected efficiently even when the photoreceptor rotates at high speed and has not yet reached smooth conveyance and recollection of the collected carrier.